Contribution of homogeneous reactions to hydrogen oxidation in catalytic microchannels

Abstract

Combustion of lean H 2/air mixtures in Pt-coated microchannels is investigated numerically in planar geometry. Examining the relative importance of hydrogen oxidation in the bulk gas as compared to surface reactions under different operating conditions is the main focus of the present work. A collocated finite-volume method is used to solve the governing equations. Detailed gas phase and surface reaction mechanisms along with a multi-component species diffusion model are used. In microchannels, due to effective heat and radical losses to the walls, the combustion characteristics are greatly influenced as the channel size is reduced. While catalytic walls help in sustaining gas phase reactions in very small channels by reducing heat losses to the walls owing to exothermic surface reactions, they also inhibit homogeneous reactions by extracting radicals due to typically high absorption rates of such species at the walls. Thus, the radical chain mechanism can be significantly altered by the presence of wall reactions, and the build-up of radical pools in the gas phase can be suppressed as a consequence. In the present work the effects of three key parameters, the channel height, the inlet mass flux and the equivalence ratio of the inlet mixture on the interaction between the gas phase and surface reactions are explored. In each case, the limiting values beyond which the gas-phase reactions become negligible compared to surface reactions are identified for lean hydrogen/air mixtures.